https://leetcode.cn/problems/is-unique-lcci/description/
class Solution {
public:
    bool isUnique(string astr) {
        if(astr.size()>26)
            return false;
        int bit=0;
        for(auto &ch:astr)
        {
            int i=ch-'a';
            if(((bit>>i)&1)==1)
                return false;
            bit|=1<<i ;
        }
        return true;
    }
};